They deliberately half-assed the parade because they don't like the way Trump treats them. Trump calls them "suckers and losers." He cuts their benefits Why would you show any loyalty (or enthusiasm) to someone who treats you like that?
More Trump quotes:
"You fucking generals. Why can't you be more like Hitler's generals?" 2017
On the burial of US Army private Vanessa Guillén:
"It doesn't cost 60,000 bucks to bury a fucking Mexican!" December 4, 2020. Trump has denied having made the remark.
"We're not going to support that loser's funeral." August, 2018. Trump has denied having made this remark.
re: John McCain, who was captured and tortured for five years:
"He's a war hero because he was captured. I like people that weren't captured, okay? I hate to tell you." July 18, 2015.
"But, you know, during the Vietnam War, I got very lucky. I had a very high lottery number." November 9, 1995.
On the 1,800 US Marines who died at Belleau Wood in France during World War I
"Suckers." November 11, 2018. Trump denies having made this remark.
On US soldiers in World War I
"Who were the good guys in this war?" November 11, 2018. Trump denies having asked this question.
"I don't get it. What was in it for them?" November 11, 2018. Trump denies having made this remark.
On disabled veterans
"Why do you bring people like that here? No one wants to see that, the wounded." September 30, 2019.
"Look, I don't want any wounded guys in the parade. This doesn't look good for me." Summer 2017.